Field of the Invention
The invention relates to a substrate structure, and more particularly to a substrate structure capable of adjusting stress and fabrication method thereof.
Description of the Related Art
A flexible display is a development trend for new-generation displays, particularly an active flexible display. Compared to conventional heavy glass substrate with brittleness, development of light flexible plastic substrate is desirable, especially active full-color TFT display panel. Currently, fabrication techniques of active flexible display comprise a-Si TFT, LPTS TFT and OTFT. Display mediums comprise EPD, ECD, LCD and EL.
Fabrication processes are divided into batch type and roll to roll. A TFT apparatus can utilize batch type fabrication processes. However, development of substrate transfer and film separation techniques is required. The flexible display must be transferred to other plastic substrates from glass. For flexible displays using roll to roll fabrication processes, new apparatuses are required and some problems caused by rolling and contact must be overcome.
The batch-type fabrication process has three methods. A PES substrate is bonded to a silicon wafer. A 7″ VGA (640×480) plastic LCD is obtained by a low-temperature a-Si TFT technique. In this manner, a transparent substrate material with heat-resistant, low thermal expansion coefficient, low light hysteresis and chemical stability is required, and combined with proper gel materials and an advanced release technique (SEC Corporation). An LPTS TFT back cover is fabricated on glass. The back cover is then removed from glass by laser annealing. The transfer technique plays an important role for this method. In the transfer technique, TFT devices with superior properties can be obtained due to no limitations by plastic substrates concerning fabrication temperature so that conventional transparent plastic substrate can be utilized (Seiko Epson Corporation). Polyimide is coated on glass to develop an a-Si TFT-EPD display. The polyimide substrate is then taken off from the glass by the transfer technique. When the polyimide substrate is directly coated on glass, the fabrication temperature is permitted to achieve 300° C. and above due to heat-resistant thereof. However, using laser annealing to remove glass substrate is also required (Philips Corporation).
A flexible TFT device with superior properties can be obtained by shaping a plastic substrate on glass and a TFT process. However, unbalanced stresses are generated on the upper and lower sides of the plastic substrate due to its asymmetrical structure, resulting in a winding flexible substrate.